hi december
I am in a similar situation.
DD is almost 17 months now and DS is almost 1 month. Her behaviour has changed a little - mostly in public i find ie wont go in the pram if i am carrying DS and throws her food etc etc.

i am atm ignoring her behaviour and when she is well behaved making the biggest doo over it as possible.

She wants to climb over me when BFing - so i do let her sit next to me and we will read a book while i BF DS and i let her stroke his head or hold his hand.

I think the more of a reaction you give him the more he will play up for your attention. So try ignoring his behaviour and praising good behaviour and find ways to involve him (as i know you will be spending a lot of time with the newborn if yours is aything like mine - ie crying 8 hrs a day!!).

Ask him to help get the nappy - or get an outfit to change him into..

when bub crys ask DS where the baby is.. lets go find him - a fun game and praise when he helps you.

Try to have meals ready for him when you are feeding so it is like a family meal time then he doesnt feel left out.

See if he wants to 'read the book to the baby' and praise when he does so...

mmm when i think of some more ill let you know.

As for dinner.. when DD refused dinner and throws her fork and meal across the room i tell her she is finished , wipe her clean and put her down. she stays down from her highchair until we are finished our dinner - we ignore her tantrums and she goes to bed without dinner. Might sound harsh but we did this for about a week and the past few nights she has been a dream at dinner time!!